Integrated distributed inductive-capacitive network

ABSTRACT

An integrated-distributed-inductive-capacitive network (100) having a high dielectric electronically-tunable semiconductor integrated capacitor. The network (100) also includes a conductive layer (126) formed on the high dielectric semiconductor integrated capacitor, to provide the distributed resistance of the network (100). External contact to the network (100) is provided via a plurality of contact terminals (122A and 122B) which are coupled to the conductive layer (126).

TECHNICAL FIELD

This invention is generally related to transmission line networks andmore particularly related to distributed inductive-capacitive networks.

BACKGROUND

High-quality capacitors are an integral part of many electricalcircuits. Capacitors are available in a variety of values with differentcharacteristics. Variable capacitors having a wide dynamic range areavailable only in bulk sizes rendering them useless for miniatureelectronic applications. Semiconductor variable capacitors, referred toas varactors, are available, however with very narrow dynamic range.Tunable electronic circuits such as filters and oscillators arepresently tuned using fixed components for coarse tune and tunablecapacitors or inductors for fine tune.

Electronic devices, and in particular communication devices, utilize avariety of circuits such as filters and oscillators that must be tunedfor proper operation. These circuits use a combination of fixed andtunable components to achieve their objectives. Fixed components areused along with tunable components to provide the tunable circuit withsufficient dynamic range. The need for fixed value components hasresulted in the inability to design and manufacture a common circuit tooperate over a desired range. Communication devices operating in aparticular band must be proliferated to accommodate different segmentsof that band due to the unavailability of alternative components. Eachboard is equipped with a different fixed value component along with avariable characteristic component to achieve the desired performancespecifications.

Another area of deficiency in electronic devices is the area ofdistributed inductive-capacitive networks. These networks areparticularly beneficial in communication devices. Present distributednetworks are also used in proliferation. That is, several circuits aredesigned and fabricated which perform the same function at differentsegments of their performance spectrum. The impact of proliferation isobvious on product cost, inventory, handling, troubleshooting, andquality. Some techniques have been employed to avoid proliferation withsome degree of success. One such technique employs components printed ona substrate and subsequently trimmed to desired spec via a well definedlaser beam. This procedure only available to high frequency applicationsis limited in range and can only reduce the number of boards performingsimilar functions and not totally eliminating the need for suchproliferation. A need is therefore clear for a tunable network having awide dynamic range with reactive characteristics.

SUMMARY OF THE INVENTION

An integrated-distributed-inductive-capacitive network is providedhaving a high dielectric electronically-tunable semiconductor integratedcapacitor. The network also includes a conductive layer formed on thehigh dielectric semiconductor integrated capacitor to provide thedistributed resistance of the network. External contact to the networkis provided via a plurality of contact terminals which are coupled tothe conductive layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a inductive-capacitive network madein accordance with the invention.

FIG. 2 is an isometric view of a inductive-capacitive network made inaccordance with the invention.

FIG. 3 shows the equivalent circuit of the network of FIG. 1.

FIG. 4 is a block diagram of an electronic circuit in accordance withthe present invention.

FIG. 5 is a block diagram of a communication device incorporating theinductive-capacitive network in accordance with the present invention.

FIG. 6 shows a waveform representing the output signal of the circuit ofFIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A voltage variable capacitor, also known as a varactor, variablecapacitance diode, or varacap, is a semiconductor device characterizedby voltage sensitive capacitance which resides in the space-chargeregion at the surface of a semiconductor bounded by an insulating layer.Varactors are very limited in their dynamic range and are useful only inhigh frequency applications where low capacitance values are used.Distributed inductive-capacitive networks are circuits havingdistributed series inductive and shunt capacitive elements. Thesecircuits have fixed capacitive elements and can not be integrated. Thisinvention provides a network having distributed inductive and capacitivecharacteristics which overcomes the deficiencies of the prior art.

Referring now to FIG. 1, there is shown a cross-sectional view of anelectronically tunable-distributed inductive-capacitive network 100, inaccordance with the principles of the present invention. The network 100may be tuned via a voltage applied to two control lines 113 and 115. Theinductive-capacitive network 100 is formed on a semiconductor substrate112 having a surface layer 114 being less heavily doped than thesubstrate 112. The surface layer 114, being less heavily doped, has ahigher resistivity than the semiconductor and serves as an area for adepletion layer to form. An insulator layer 116 is applied over thesurface layer 114. At least one metal plate 118 is formed on theinsulator layer 116. The metal plate 118 provides the external contactto the reactance component of the network 100 via a contact pin 113.More metal plates along with contacts may be formed on the insulatorlayer 116 to provide further control over the distributed capacitance ofthe network 100. A conductive layer 126 is formed on the insulatinglayer 116 surrounding the metal plate 118 to provide the inductiveelement of the network 100. This conductive layer 126 may be anymaterial having conductive characteristics desired by the requirementsof the network 100. Some suitable materials include nickel chromium,aluminum, gold, titanium, tungsten, vanadium, and alloys thereof. Twometal plates 122A and 122B are shown on the conductive layer 126 toprovide external contact with the inductive component of the network100.

The insulator layer 116 is preferably Zirconium Titanate (ZrTiO₄)applied in a thickness from 300 Ångstroms to 1000 1000 Ångstroms, butthicknesses from 100 Ångstroms to 2 microns have been found to besuitable. The material employed as the dielectric or insulating layershould have a dielectric constant much greater than that of thesemiconductor. Examples of suitable materials that may be used for thispurpose are to be found in TABLE 1 below:

                  TABLE 1                                                         ______________________________________                                        tantalum pentoxide    Ta.sub.2 O.sub.5                                        niobium pentoxide     Nb.sub.2 O.sub.5                                        zirconium oxide       ZrO.sub.2                                               titanium dioxide      TiO.sub.2                                               zirconium titanate    ZrTiO.sub.4                                             strontium titanate    SrTiO.sub.3                                             barium titanate       BaTiO.sub.3                                             lead titanate         PbTiO.sub.3                                             barium tetratitanate  Ba.sub.2 Ti.sub.9 O.sub.20                              barium neodymium titanate                                                                           BaNd.sub.2 Ti.sub.5 O.sub.14                            lead-zirconium titanate                                                                             Pb(Zr,Ti)O.sub.3                                        lead-lanthanum zirconium titanate                                                                   (Pb,La)(Zr,Ti)O.sub.3                                   lithium niobate       LiNbO.sub.3                                             strontium-barium niobate                                                                            (Sr,Ba)Nb.sub.2 O.sub.6                                 ______________________________________                                    

Oxides of additional elements such as molybdenum, tungsten and vanadiummay also be expected to be useful, either alone or in combination withother elements.

When an appropriate reverse bias is applied to the metal electrode 118,mobile minority charge carriers are attracted to a semiconductorinsulator interface 119, forming a space-charge or depletion layer 120,which extends for some distance into the conductor 114. This depletionlayer 120 behaves as a variable width capacitor which is electrically inseries with the capacitor formed by the insulator layer 116. These twoseries capacitors serve to create a net capacitance effect that isaffected by the changes of each individual capacitor. The electrode biasvoltage controls the width of the depletion layer 120 from zero at theaccumulation threshold to a maximum thickness at the inversion thresholdand thereby varies the total capacitance of the device. The insulatorlayer 116 serves to provide the spacing between the top electrode 118and the depletion layer 120. The depletion layer 120 is a transientlayer formed when the bias voltage is applied to the capacitor throughinput contacts 113 and 115. The depletion layer 120, hence thedistributed capacitance, may be reduced or disappear when the appliedvoltage field is varied or removed. Although shown in the drawing as adistinct feature, the depletion layer 120 should not be regarded as apermanent mechanical feature of the network 100. The operation theorydescribed herein is similar to that found in operation ofmetal-oxide-semiconductor capacitors.

At the inversion threshold voltage, enough charge carriers have beenattracted to the semiconductor interface such that an inversion layer isformed. Increasing the voltage bias increases the width of the inversionlayer, until the layer reaches a maximum width, beyond which thedepletion layer cannot be substantially increased by increasingelectrode bias voltage. The maximum depletion width is determined by theconcentration of the impurity dopant near the semiconductor surface ontowhich the insulator layer 116 has been deposited. Dopants such asphosphorous, antimony, boron and arsenic will be recognized by thoseskilled in the art to be useful with silicon substrates. Othersemiconductor substrates, such as gallium arsenide may also be utilizedto form a VVC in accordance with the invention.

The lower the doping, the larger the maximum depletion layer thickness,and thus, the lower minimum capacitance which can be achieved. Thethickness of a less heavily doped surface layer may be chosen to beequal to or slightly greater than this maximum depletion width in orderto minimize the series resistance of the device while maximizing thecapacitance change.

Formation of an improved voltage tunable inductive capacitive network ishighly dependent upon the choice of the material comprising theinsulator layer 116. By choosing a material with a much larger relativedielectric constant than the semiconductor depletion layer 120, a largerratio of maximum-to-minimum distributed capacitance will be obtained.The larger the insulator's dielectric constant, the larger thecapacitance ratio in capacitance per unit area will be for a giveninsulator thickness.

Many materials with very high dielectric constants have ferroelectricproperties which are not desirable for high frequency devices. Thepolarization for a ferroelectric material has a hysteresis loop, ormemory, whereby a residue polarization remains after an applied biasvoltage has been removed. Thus, a residual depletion layer would alsoremain and thereby limit the capacitance ration which may be obtained.These materials would be best utilized in lower frequency applications.

A low-loss, non-ferroelectric insulator layer is required for highfrequency applications, specifically those for use in radio transmittingand receiving, and especially for tunable high-Q filters. ZirconiumTitanate (ZrTiO₄) is one suitable non-ferroelectric material with a highrelative dielectric constant (K_(r) is approximately equal to 40) andlow dielectric loss. By comparison, the relative dielectric constant ofsilicon dioxide (used in conventional MOS capacitors) is 3.9. Thedielectric constant of the depletion layer in silicon is 11.7 and thedielectric constant of the depletion layer in germanium is 15.7. It canbe easily seen that the dielectric constant of the zirconium titanateand the aforementioned materials in Table 1 is much larger than that ofsilicon dioxide. Therefore, an improved capacitor having highercapacitance ratio can be fabricated. Thin films of zirconium titanatecan be formed by any of several techniques, including but notnecessarily limited to, sputtering, evaporation, chemical vapordeposition, ion beam or plasma enhanced processes, sol-gel, and othersolution chemistry processes.

By choosing an insulator with a much larger relative dielectric constantthan a semiconductor depletion layer, a larger ratio between the maximumcapacitance at zero depletion layer thickness and the minimumcapacitance at the inversion threshold can be achieved. This strategyhas been largely overlooked because the theory ofMetal-Insulator-Semiconductor (MIS) capacitors was developed with asilicon dioxide insulator on silicon. Because the maximum width of thedepletion layer in an MIS capacitor is limited by the formation of aninversion layer, the capacitance change which can be achieved with a lowdielectric constant material, such as silicon dioxide, is less than orcomparable to what can be achieved by varying the depletion width arounda PN junction.

Referring now to FIG. 2, an isometric view of the network 100 is shown.The various elements of the network 100 are shown in this view to betterillustrate the preferred embodiment of the present invention. Thepresentation of this view is meant only to enhance the understanding ofthe layers involved in the construction of the network 100. It is not inany fashion meant to imply, directly or otherwise, a limitation on thepresent invention. The location of the film contacts 122A, 122B, and 128is critical to the radio frequency (RF) operation of the network 100.The location of the metal plate 118 serves to provide substantiallylocalized capacitance variations as deemed necessary by the objectivesof a particular application. Note that the metal plate 118 may beeliminated in applications where localized capacitance variation is notdesired. This is due to the entire area of the layer 126 beingconductive. A DC voltage applied to either contacts 122A or 122B will beevenly available on the entire surface of the conductor layer 126forming a uniform depletion region thereunder. As mentioned before,although the contacts 122A and 122B appear redundant in application ofDC signals, their individual location is significant to the proper RFoperation of the network 100. In general, various performance objectivesmay be met by placing these contacts 122A, 122B, and 118 in differentplaces. Furthermore, more contact plates may be added on the conductivelayer 126 to provide additional external contact with the network 100.

The depletion region 120 is shown to be located substantially beneaththe metal layer 118. As the number of metal plates, such as 118 isincreased, to achieve a higher degree of reactance distribution, thedepletion regions formed under each contact area will function as aseparate shunt capacitor. It can therefore be seen that the two plates122A and 122B represent the two ends of a inductor while the plate 118provides the control input of an electronically tunable capacitiveelement in shunt with the inductor of the network 100. This is madeclearer by referring to FIG. 3.

Referring to FIG. 3, an equivalent electrical circuit diagram 300 of thenetwork 100 is shown. The diagram 300 depicts an ideal case neglectingthe dielectric and conductor losses, however small. Operationally, thenetwork 100 is a number of series inductance and capacitance per unitlength. The series inductance per unit length value is a function of themetal electrode width (W) and thickness (t). The shunt capacitance isdetermined by the following relationship which assumes parallel platecapacitance and neglects any fringe capacitance. ##EQU1## where:A=Electrode Area

ε_(r1) =Relative Dielectric Constant of the fixed insulator

ε_(r2) =Relative Dielectric Constant of the variable depletion layer 120

t₁ =Thickness of the insulator layer 116

t₂ =Thickness of the depletion layer 120.

For dielectric materials with a relative permittivity of 1, theinductance per unit length (L) remains constant independent of thedepletion layer thickness. The variation of the capacitance and theeffective dielectric constant will effect the transmission linecharacteristic impedance and propagation velocity by the followingrelation: ##EQU2## where: Z₀ is the characteristic impedance

V is the propagation velocity

c is the speed of light=3×10⁸ meters/second

ε_(reff) is the effective dielectric constant of the transmission linestructure.

It is significant to note that the network 100 makes a variety ofpreviously unrealizable electronic circuits possible. Each of the shuntcapacitors may be realized by a separate depletion region formed on thenetwork 100 via strategically located contact plates. The control forthese capacitors is provided via DC voltage(s) applied at one or anumber of control inputs. The application of this network is widespread. FIG. 4 shows one such application.

Referring to now FIG. 4, a schematic diagram of a typical circuit 400taking advantage of the distributed network 100 is shown. The circuit400 is a variable transmission zero circuit which includes an oscillator402 coupled to the network 100 via a coupler including a resistor 404and a DC blocking capacitor 406. The oscillator 402 used in thisembodiment represents a means of producing an input signal. This inputsignal may be provided via an antenna at the input of a receiver.Positive bias or otherwise the control voltage for the network 100 iscoupled through an isolation resistor 410. The resistor 410 providesradio frequency isolation between the bias line and the network 100. Abypass capacitor 414 provides additional filtering of the undesiredradio frequency signals thereby assisting in the high frequencyisolation of the network 100 from the bias line. The network 100 iscoupled to a load 416 via a DC blocking capacitor 412. The twocapacitors 406 and 412 provide the necessary DC blocking at the twoports of the network 100 in order to minimize input and outputinterferences on the operating parameters of the network 100 which arecontrolled via the DC signal on the bias line.

The operation of the circuit 400 having a number of zeros can be bestexplained by referring to the signal diagram 602 of FIG. 6 which showsthe frequency-amplitude transfer function of the circuit 400. As can beseen several zeros 604 and 606 are available at the output of thenetwork 100. The frequency of these valleys 604 and 606 could be readilyvaried by the bias voltages applied at the bias line. In general thefrequency of the transmission zeros 604 and 606 is determined by:##EQU3##

It can simply be seen that an electronically tunable band reject filteris easily accomplished having variable reject (resonance) frequencies.Since the tuning of the zeros is fully electronic, there is no need todesign a filter for each separate band of operation. Rather, with theability to change its operating frequency, the same filter may be usedfor any band of operation. Note that although the frequency of operationof a filter using the circuit 400 is shown to be variable, otherperformance characteristics such as bandwidth and phase performance maybe tuned using the network 100 in other strategic locations.

The circuit 400 is shown here to illustrate the significance of thepresent invention, for this is a particularly difficult topology torealize without discrete components. It is obvious that any otherelectronic circuit having an electronically tunable network can benefitfrom this invention. Some such circuits are oscillators, filters ofdifferent characteristics, switches, etc. A variable transmission zerois particularly described in the preferred embodiment to demonstrate thebenefits of the present invention in accommodating the complexities thatare involved in achieving desirable frequency and phase performance forsuch circuits.

Referring now to FIG. 5, a block diagram of a receiver 500 in accordancewith the principles of the present invention is shown. The receiver 500includes an antenna 502 by which radio frequency signals are receivedand applied to a filter 504 for filtering. The filter 504 is anintegrated electronically tunable distributed inductive-capacitivenetwork constructed using the principles of the present invention. Thefrequency of operation of the filter 504 is controlled by a controller516. Coupled to the controller 516 is a memory block 514 that providesthe controller 516 with frequency information and other informationrelevant to the operation of the receiver 500. This information istranslated at the controller 516 and converted to relative values usedto command the frequency of operation of the filter 504. Filteredsignals are amplified by an RF amplifier 506 and applied to ademodulator 508. The demodulator 508 uses the oscillator signal of anoscillator 518. The oscillator 518 determines the operating frequency ofthe receiver 500. This oscillator 518 includes a inductive-capacitivenetwork similar to 100. Once again, the controller 516 controls theoperating frequency of the oscillator 518 by controlling the biasingvoltage of its electronically tunable network. The output of thedemodulator 508, is coupled to a power amplifier 510 via an audio filter509. The filtering performance of the filter 509 is once againcontrolled by the controller 516. Filtered signals at the output of thefilter 509 are amplified by the amplifier 510 and applied to a speaker512. Information on the frequency of operation of the filters 504 and509 and the oscillator 518 are stored in the memory 514 and communicatedto the controller 516 upon request.

The ability to control the frequency of operation of the filter 504renders the integration of all the elements of the receiver 500 on anintegrated circuit more feasible. The predicament in fabricating asingle chip receiver can now be removed by eliminating the need fordiscrete components that were not realizable in semiconductor. With thenetwork 100 fully integratable it is now possible to electronicallycontrol the operation of many electronic circuits previously notpossible. A significant benefit of this is the elimination of the needto proliferate receivers assemblies to cover a desired operating range.

A significant benefit of integrating a transmission line is its matchingof component parameters. It is well known that an integrated circuitwill have similar component tolerance variation across a given circuitfor the same type of components. For example, if the fabrication processyields a capacitance per square of electrode that is low by a similarpercentage. This would result in a control voltage tracking across acircuit. Considering the circuit 400 with the zeros 604 and 606 isolatedby a 90% phase shift, the voltage to produce a given f_(Z) will beconsiderably closer with an integrated implementation, compared to adiscrete implementation. In other words, the integrated componentvariation have a larger correlation than discrete implementations.

Another benefit of integrating a filter along with its reactancecomponents is that the reactance component (VVC) can be compensated fortemperature, humidity, or other environmental conditions. Additionalsensors can be implemented with the VVC that have same processingvariations as the VVC, rendering them significantly more accurate. Thiscan be used to overcome the processing and environmental circuitperformance variations to maintain a stable circuit performance.

Although the focus of this embodiment has been on transmission zerousing a high dielectric integrated tunable network, it is wellunderstood that a variety of electronic circuits may be implementedusing the principles of the present invention. Thus, a variety offilters such as low pass, high pass, band reject, etc. may beimplemented using these principles. Other circuits such as tunableinductor simulation using capacitive components is well within the scopeof the operation of the network 100.

In summary, it may be seen that favorable results may be obtained fromutilizing this structure, including integration of wide dynamic rangetunable circuits, prevention of contact between the top and bottomelectrodes through conducting grain boundaries and voids, reduction ofnonhomogeneous and field enhancing regions, lower loss, higher Q,improvement in film resistivity, improved electrical breakdown, andimproved storage charge characteristics. The foregoing examples areintended to serve as an illustration of the preferred embodiment of theinvention. Accordingly, it is not intended that the invention be limitedexcept as by the appended claims herein.

What is claimed is:
 1. An integrated-distributed-inductive-capacitivenetwork, comprising:a Zirconium Titinate integrated capacitorincluding:a semiconductor having a substrate and a layer ofsemi-conductive material of a higher resistivity than the substrate; adepletion layer formed in the high resistivity layer; an insulatinglayer formed on the high resistivity layer, said insulating layer beinga metal oxide having a dielectric constant greater than the dielectricconstant of the semiconductor; an electrode formed on the dielectriclayer; a conductive layer formed on the high dielectric semiconductorintegrated capacitor; and a plurality of electrical contact terminalscoupled to the conductive layer.
 2. Anintegrated-distributed-inductive-capacitive network, comprising:aZirconium Titanate integrated capacitor including: a semiconductorhaving a substrate and a layer of semi-conductive material of a higherresistivity than the substrate; a depletion layer formed in the higherresistivity layer; an insulating layer formed on the high resistivitylayer, the insulating layer comprising a metal oxide having a dielectricconstant greater than the dielectric constant of the semiconductor; ametal electrode formed on a portion of the insulating layer; aconductive layer formed on the insulating layer; and a plurality ofmetal electrodes formed on the conductive layer.
 3. An integrateddistributed inductive-capacitive network having a control input, acommon line and two signal lines, comprising:a Zirconium Titinateintegrated capacitor, including:a semiconductor having a substrate and alayer of semi-conductive material of a higher resistivity than thesubstrate; a depletion layer formed in the high resistivity layer; aninsulating layer formed on the high resistivity layer, said insulatinglayer being a metal oxide having a dielectric constant greater than thedielectric constant of the semiconductor; an electrode formed on thedielectric layer and coupled to the control line; a first contact platecoupled to the common line; a conductive layer formed over theintegrated capacitor; and a plurality of contact terminals coupled tothe conductive layer and providing the two signal lines.
 4. Acommunication device, comprising:receiver means for receiving radiocommunication signals, the means for receiving having at least oneintegrated electronically tunable distributed inductive-capacitivenetwork, the integrated network including: a Zirconium Titinateintegrated capacitor, including:a semiconductor having a substrate and alayer of semi-conductive material of a higher resistivity than thesubstrate; a depletion layer formed in the high resistivity layer; aninsulating layer formed on the high resistivity layer, said insulatinglayer being a metal oxide having a dielectric constant greater than thedielectric constant of the semiconductor; an electrode formed on thedielectric layer; a conductive layer formed on the high dielectricsemiconductor integrated capacitor; and a plurality of contact terminalscoupled to the conductive layer.